Hi mark
As a linux user you are probably more technically sophisticated (and demanding) than the average so you have some additional options open.
Basically the iPad does nothing in terms of file access and sharing out of the box (except via USB, which for a wi fi device is kinda "fail" right?).
However there are some OK apps for file sharing with different methods, and some more advanced options you can play with in conjunction on your linux box.
Most of the apps are httpd/WebDav or ftp/sftp clients. So if you set up apache on your machine, and point it to your home directory you will have access to all your files. If you enable WebDAV on it, you will be able to write back them too. This should have negligible performance impact on any reasonable PC from the last 2-3 years.
Now the apps: The best to consider are:
Air Sharing: Supports ftp, sftp, webdav, dropbox, idisk and all the other cloud storage types. However you can set up an ftp or WebDAV server on your linux bo and you're off. Speeds are so-so.
Filer: nice and simple, has an http client to download files (I think it also supports WebDav & dropbox). Fastest of the bunch.
FileBrowser: this is a SAMBA client, so the advantage is it finds windows shares as well or you can configure SMB on your linux systems. ITs a bit buggy, sometimes doesn't list diectory contents if there are many files in them and speeds vary between filetypes (!). It kind feels like a SAMBA implementation written as a school computing project but its OK.
I find Filer the easiest to use, air sharing the most complete. A lot of people use GoodReader - it's ok too but its noit a dedicated file sharing tool so its UI can be a bit awkward.
Basically you cannot avoid doing some extra config on your PCs, the trouble is the iPad wasn't made for people like us, but its still a nice device.